Japanese Patent Laid-Open No. 2005-197674 (called “Patent Document 1” hereinafter) discloses a photoelectric conversion apparatus in which a P-type well to be disposed below an N-type charge accumulating area is described as being configured of multiple impurity areas 4A to 4D having differing depths, as shown in FIG. 1 of that document. The density of the impurity area 4A, which is the deepest of the multiple impurity areas 4A to 4D, is greater than the densities of the other impurity areas. Therefore, according to Patent Document 1, a potential barrier for electrons can be formed in the depth direction of the substrate, which makes it possible to guide the carrier to a photodiode in the depth direction efficiently and without loss, which in turn makes it possible to improve the sensitivity.
Incidentally, there is demand for pixel miniaturization and pixel sensitivity improvement in photoelectric conversion apparatuses. It is conceivable that miniaturizing the pixels in the photoelectric conversion apparatus disclosed in Patent Document 1 will also bring adjacent photodiodes closer to each other.
Patent Document 1 discloses forming the multiple impurity areas 4A to 4D by implanting boron, first in deep areas and moving sequentially to shallow areas, and then executing a heat treatment of 950° C. or less. When executing such heat treatment after forming the multiple impurity areas 4A to 4D, boron tends to diffuse even under heat treatments of 950° C. or less, and therefore the impurity density peaks in the respective impurity areas 4A to 4D drop easily. As a result, the slope of the potential along the substrate depth direction broadens, resulting in the possibility that the electrons will not reach the photodiodes of pixels in which electrons are produced and will instead reach adjacent pixels based on the mechanics of diffusion. In color sensors whose pixels have different spectral transmission characteristics from one another, this phenomenon results in a problem called “color mixture”, whereby light of a certain color leaks into pixels that respond to a different color. In monochromatic sensors, this phenomenon appears as a drop in the MTF. This problem is apparent particularly when the distance between adjacent photodiodes (photoelectric conversion portions) is low.